High-frequency switching type conversion device

ABSTRACT

Provided is a conversion device that converts DC power provided from a DC power supply, to AC power and supplies the AC power to a load, the conversion device including: a filter circuit connected to the load and including an AC reactor and a first capacitor; a DC/AC inverter connected to the load via the filter circuit; a DC/DC converter provided between the DC power supply and the DC/AC inverter; a second capacitor provided between the DC/AC inverter and the DC/DC converter; and a control unit configured to set a current target value for the DC/DC converter to thereby be synchronized with current of the AC power, based on voltage of the AC power, voltage variation due to current flowing through the AC reactor and an impedance thereof, reactive currents respectively flowing through the first capacitor and the second capacitor, and voltage of the DC power.

TECHNICAL FIELD

The present invention relates to a conversion device that performsconversion from DC to AC or conversion from AC to DC.

BACKGROUND ART

Conversion devices that convert DC voltage outputted from a storagebattery to AC voltage and provides the AC voltage to a load are oftenused as a backup power supply such as a UPS (Uninterruptible PowerSupply) (see, for example, Patent Literature 1 (FIG. 1)). Such aconversion device includes a DC/DC converter for stepping up voltage ofthe storage battery and an inverter for performing conversion from DC toAC. The conversion device is capable of bidirectional operation, andnormally, converts AC voltage outputted from an AC power supply such asa commercial power supply, to DC voltage adapted for charging, andcharges the storage battery. In this case, the inverter operates as anAC/DC converter, and the DC/DC converter performs step-down operation.

A conversion device (power conditioner) is used also for converting DCpower obtained from a DC power supply such as photovoltaic powergeneration to AC power and performing system interconnection with an ACpower system (see, for example, Patent Literature 2).

CITATION LIST Patent Literature

PATENT LITERATURE 1: Japanese Laid-Open Patent Publication No.2003-348768

PATENT LITERATURE 2: Japanese Laid-Open Patent Publication No.2000-152651

SUMMARY OF INVENTION Technical Problem

In the above conventional conversion device, the AC/DC converter and theDC/DC converter are both composed of switching elements, and constantlyperform high-speed switching. Such switching elements are accompaniedwith slight switching loss. Although loss in one switching is slight,high-frequency switching of a plurality of switching elements causesswitching loss that cannot be neglected as a whole. The switching lossnaturally leads to power loss. On the other hand, Patent Literature 2discloses a control method for reducing loss, but there is a problemthat a loss reduction effect is not sufficiently obtained by thiscontrol method alone, and the AC waveform is distorted.

In view of the above problems, an object of the present invention is toachieve a high conversion efficiency by reducing switching loss in aconversion device, and achieve an AC waveform with small distortion.

Solution to Problem

A conversion device of the present invention is a conversion device thatconverts DC power provided from a DC power supply, to AC power andsupplies the AC power to a load, the conversion device including: afilter circuit connected to the load and including an AC reactor and afirst capacitor; a DC/AC inverter connected to the load via the filtercircuit; a DC/DC converter provided between the DC power supply and theDC/AC inverter; a second capacitor provided between the DC/AC inverterand the DC/DC converter; and a control unit configured to set a currenttarget value for the DC/DC converter to thereby be synchronized withcurrent of the AC power, based on voltage of the AC power, voltagevariation due to current flowing through the AC reactor and an impedancethereof, reactive currents respectively flowing through the firstcapacitor and the second capacitor, and voltage of the DC power.

Advantageous Effects of Invention

The conversion device of the present invention can achieve a highconversion efficiency and an AC waveform with small distortion at thesame time.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing an example of a system including aninverter device according to the first embodiment.

FIG. 2 is an example of a circuit diagram of the inverter device.

FIG. 3 is a block diagram of a control unit.

FIG. 4 is a graph showing an example of a simulation result of temporalvariations in a DC input voltage detection value and a step-up circuitcurrent detection value.

FIG. 5 is a diagram showing a manner in which an averaging processingunit averages a DC input voltage detection value Vg.

FIG. 6 is a control block diagram for explaining a control process by acontrol processing unit.

FIG. 7 is a flowchart showing a control process for a step-up circuitand an inverter circuit.

FIG. 8 is graphs in which (a) shows an example of a simulation result ofa step-up circuit current target value calculated in a feedback controlby the control processing unit, and a step-up circuit current detectionvalue obtained when control is performed in accordance with the step-upcircuit current target value, and (b) shows an example of a simulationresult of a step-up circuit voltage target value calculated in thefeedback control by the control processing unit, and a step-up circuitvoltage detection value obtained when control is performed in accordancewith the step-up circuit voltage target value.

FIG. 9 is a diagram showing an example of an inverter output voltagetarget value.

FIG. 10 is graphs in which (a) shows comparison between a step-upcircuit carrier wave and a step-up circuit reference wave, and (b) showsa drive waveform for driving a switching element Qb, generated by astep-up circuit control unit.

FIG. 11 is graphs in which (a) shows comparison between an invertercircuit carrier wave and an inverter circuit reference wave, (b) shows adrive waveform for driving a switching element Q1, generated by aninverter circuit control unit, and (c) shows a drive waveform fordriving a switching element Q3, generated by the inverter circuitcontrol unit.

FIG. 12 is a diagram showing examples of reference waves and drivewaveforms for switching elements, and an example of a current waveformof AC power outputted from the inverter device.

FIG. 13 is graphs in which (a) shows voltage waveforms of AC voltageoutputted from the inverter circuit, a commercial power system, andvoltage between both ends of an AC reactor, and (b) shows a waveform ofcurrent flowing in the AC reactor.

FIG. 14 is an example of a circuit diagram of an inverter deviceaccording to the second embodiment.

FIG. 15 is a graph showing comparison between an inverter circuitcarrier wave in the second embodiment and a reference wave.

FIG. 16 is a diagram showing examples of reference waves and drivewaveforms for switching elements Qb and Q1 to Q4, and an example of acurrent waveform of AC power outputted from the inverter device, in thesecond embodiment.

FIG. 17 is an example of a circuit diagram of an inverter device 1according to the third embodiment.

FIG. 18 is a diagram showing examples of reference waves and drivewaveforms for switching elements, and an example of a current waveformof AC power outputted from the inverter device, in the third embodiment.

FIG. 19 is a block diagram showing an example of a power storage systemincluding an AC-to-DC conversion device.

FIG. 20 is an example of a circuit diagram of the conversion device.

FIG. 21 is a voltage waveform diagram conceptually showing operation ofthe conversion device.

FIG. 22 is examples of AC output waveforms in the first embodiment and acomparative example.

DESCRIPTION OF EMBODIMENTS Summary of Embodiments

Summary of the embodiments of the present invention includes at leastthe following.

(1) This provides a conversion device that converts DC power providedfrom a DC power supply, to AC power and supplies the AC power to a load,the conversion device including: a filter circuit connected to the loadand including an AC reactor and a first capacitor; a DC/AC inverterconnected to the load via the filter circuit; a DC/DC converter providedbetween the DC power supply and the DC/AC inverter; a second capacitorprovided between the DC/AC inverter and the DC/DC converter; and acontrol unit configured to set a current target value for the DC/DCconverter to thereby be synchronized with current of the AC power, basedon voltage of the AC power, voltage variation due to current flowingthrough the AC reactor and an impedance thereof, reactive currentsrespectively flowing through the first capacitor and the secondcapacitor, and voltage of the DC power.

In the conversion device configured as in the above (1), the DC/ACinverter and the DC/DC converter each perform high-frequency switching aminimum necessary number of times. In addition, the AC/DC converteroperates in a region other than the peak of the absolute value of ACvoltage and the vicinity thereof, and the DC/DC converter operates in aregion other than the zero cross point of AC voltage and the vicinitythereof. Therefore, in high-frequency switching, voltages applied to thesemiconductor elements of each converter and the reactor are relativelyreduced. This also contributes to reduction in switching loss in thesemiconductor elements and iron loss in the reactor. Thus, the entireloss in the conversion device can be reduced.

In order to achieve the “minimum necessary number of times” as describedabove, ideally, it is preferable that the DC/AC inverter and the DC/DCconverter alternately perform high-frequency switching so that theirrespective periods of high-frequency switching do not overlap eachother. However, in practice, even if both periods slightly overlap eachother, as long as a stop period is provided for each of the DC/ACinverter and the DC/DC converter, the loss can be reduced, leading toenhancement in the efficiency.

Based on voltage of AC power, voltage variation due to current flowingthrough the AC reactor and the impedance thereof, reactive currentsflowing through the first and second capacitors, and voltage of DCpower, the current target value for the DC/DC converter is set to besynchronized with current of AC power, whereby AC power with nodistortion can be constantly generated. Particularly, in a case wherethe load is interconnected with an AC system, current that is constantlysynchronized with the system voltage (or controlled at a given phaseangle relative to the system voltage) and has no distortion can flow toor from the AC system, irrespective of variation in the voltage,frequency, and output current of the AC power supply.

(2) In the conversion device of (1), preferably, in a case where anoutput current target value for the load is Ia*, an electrostaticcapacitance of the first capacitor is Ca, a voltage value of the ACpower is Va, voltage on the DC power supply side is V_(DC), and anLaplace operator is s, the control unit sets an AC output current targetvalue Iinv* for the DC/AC inverter at a circuit connection point betweenthe filter circuit and the DC/AC inverter, as follows:Iinv*=Ia*+sCaVa,

in a case where an impedance of the AC reactor is Za, the control unitsets an AC output voltage target value Vinv* for the DC/AC inverter atthe circuit connection point, as follows:Vinv*=Va+ZaIinv*,

the control unit sets the greater one of the voltage V_(DC) and anabsolute value of the AC output voltage target value Vinv* for the DC/ACinverter, as an output voltage target value Vo* for the DC/DC converter,and

in a case where an electrostatic capacitance of the second capacitor isC, the control unit sets a current target value Iin* for the DC/DCconverter, as follows:Iin*={(Iinv*×Vinv*)+(sCVo*)×Vo*}/V _(DC).

The conversion device of the above (2) is an example showing a morespecific control manner for achieving the conversion device of (1). Inthe current target value Iin* for the DC/DC converter, voltage of ACpower, voltage variation due to current flowing through the AC reactorand the impedance thereof, reactive currents flowing through the firstand second capacitors, and voltage of DC power are all reflected.Therefore, irrespective of variation in voltage of the DC power supplyor variation in the AC output current, power synchronized with the ACoutput current can be constantly outputted. Therefore, the DC/DCconverter and the DC/AC inverter can perform conversion from AC to DCwhile performing high-frequency switching a minimum necessary number oftimes. As a result, switching loss in semiconductor switching elementsand iron loss in an AC reactor and a DC reactor can be greatly reduced,and a high conversion efficiency can be achieved. Further, outputted ACpower has a high quality, and current with distortion that issufficiently small for interconnection with the commercial system can beobtained.

Instead of Laplace operator s, in a case of using a derivative withrespect to time t, the above expression is represented as follows.Iinv*=Ia*−Ca×(dVa/dt)Iin*={(Iinv*×Vinv*)+C×(dVo*/dt)×Vo*}/V _(DC)

By measuring power loss P_(LOSS) in the conversion device in advance,the current target value Iin* can also be represented by the followingexpression.Iin*={(Iinv*×Vinv*)+C×(dVo*/dt)×Vo*+P _(LOSS) }/V _(DC)

In this case, it is possible to calculate the value of Iin* morestrictly by considering the power loss P_(LOSS).

(3) In the conversion device of (2), the DC/DC converter may include aDC reactor, and in a case where voltage of the DC power supply is Vg, animpedance of the DC reactor is Z, and a current value of the DC/DCconverter is Iin, (Vg−ZIin) may be used as the voltage V_(DC).

(4) As the current value Iin of the DC/DC converter, a detection value(a current detection value of the DC reactor) of a current sensor or acalculation value obtained by Iinv*×Vinv*/Vg may be used.

In the cases of the above (3) and (4), since voltage drop due to thecurrent and impedance of the DC reactor is also considered, it ispossible to constantly perform accurate control irrespective ofvariation in current flowing through the DC/DC converter.

(5) In the conversion device of any one of (1) to (4), the DC/ACinverter may be controlled based on comparison between a reference valuebased on a target value and a detection value of AC output current ofthe DC/AC inverter, and an output voltage target value for the DC/DCconverter, and the DC/DC converter may be controlled based on comparisonbetween a reference value based on a current target value and adetection value of the DC/DC converter, and the output voltage targetvalue for the DC/DC converter.

Thus, the DC/AC inverter and the DC/DC converter are controlled based onthe same voltage target value, whereby the distortion rate of the ACoutput current can be reduced.

(6) An AC power supply may be connected in parallel to the load.

The conversion device configured as in the above (6) converts powersupplied from the DC power supply, to AC power, performs paralleloperation interconnected with the AC power supply such as a commercialpower system, and thereby can provide the AC power supply to the load.

(7) The DC power supply may be a DC load, the load may be an AC powersupply, and power may be supplied from the AC power supply to the DCload.

The conversion device configured as in the above (7) can performconversion from AC to DC.

In conversion from AC to DC, the DC/AC inverter operates as an AC/DCconverter. The DC/DC converter operates as a DC/DC converter in whichcurrent flows in a direction opposite to the direction in conversionfrom DC to AC. Also in conversion from AC to DC, the AC/DC converter andthe DC/DC converter perform high-frequency switching a minimum necessarynumber of times. Therefore, switching loss in the semiconductorswitching elements and iron loss in the AC reactor and the DC reactorcan be greatly reduced, and a high conversion efficiency can beachieved.

In the calculation expressions in (2), if the phase of the outputcurrent target value Ia* is inverted, expressions for giving a targetvalue in conversion from AC to DC are obtained. In this case, thecurrent target value Iin* for the DC/DC converter is negative relativeto the DC voltage Vg. Therefore, actually, the conversion devicesdescribed in (1) to (7) allow conversion from DC to AC and conversionfrom AC to DC by a common device.

(8) It is desirable that an SiC element is used for at least one ofsemiconductor switching elements included in the DC/DC converter and theDC/AC inverter.

In the conversion device described in any one of (1) to (7), switchingloss in the semiconductor elements and iron loss in the DC reactor andthe AC reactor can be reduced by decrease in the number of times ofhigh-frequency switching, but conduction loss in the semiconductorelements cannot be reduced. In this regard, using SiC elements as thesemiconductor elements enables reduction in the conduction loss.Therefore, if SiC elements are used for the conversion device of any oneof (1) to (7), a high conversion efficiency can be achieved by thesynergistic effect therebetween.

Details of Embodiments

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings.

<<DC-to-AC Conversion Device with System Interconnection Function>>

First, a DC-to-AC conversion device with a system interconnectionfunction (hereinafter, simply referred to as an inverter device) will bedescribed in detail.

1. First Embodiment

[1.1 Overall Configuration]

FIG. 1 is a block diagram showing an example of a system including aninverter device according to the first embodiment. In FIG. 1, aphotovoltaic panel 2 as a DC power supply is connected to an input endof an inverter device 1, and an AC commercial power system 3 (AC system)is connected to an output end of the inverter device 1. This systemperforms interconnection operation to convert DC power generated by thephotovoltaic panel 2 to AC power and output the AC power to thecommercial power system 3.

The inverter device 1 includes a step-up circuit (DC/DC converter) 10which receives DC power outputted from the photovoltaic panel 2, aninverter circuit (DC/AC inverter) 11 which converts power given from thestep-up circuit 10 to AC power and outputs the AC power to thecommercial power system 3, and a control unit 12 which controlsoperations of these circuits 10 and 11.

FIG. 2 is an example of a circuit diagram of the inverter device 1.

The step-up circuit 10 includes a DC reactor 15, a diode 16, and aswitching element Qb composed of an Insulated Gate Bipolar Transistor(IGBT) or the like, to form a step-up chopper circuit.

On an input side of the step-up circuit 10, a first voltage sensor 17, afirst current sensor 18, and a capacitor 26 for smoothing are provided.

The first voltage sensor 17 detects a DC input voltage detection valueVg (DC input voltage value) of DC power outputted from the photovoltaicpanel 2 and then inputted to the step-up circuit 10, and outputs the DCinput voltage detection value Vg to the control unit 12. The firstcurrent sensor 18 detects a step-up circuit current detection value Iin(DC input current value) of current flowing in the DC reactor 15, andoutputs the step-up circuit current detection value Iin to the controlunit 12. In order to detect a DC input current detection value Ig, acurrent sensor may be further provided at a stage preceding thecapacitor 26.

The control unit 12 has a function of calculating input power Pin fromthe DC input voltage detection value Vg and the step-up circuit currentdetection value Iin and performing maximum power point tracking (MPPT)control for the photovoltaic panel 2.

As described below, the switching element Qb of the step-up circuit 10is controlled so as to minimize a sum of the number of times theswitching element Qb performs switching operation and the number oftimes the inverter circuit 11 performs switching operation, and a stopperiod arises. Therefore, during a period in which switching operationis performed in the step-up circuit 10, the step-up circuit 10 outputsstepped-up power to the inverter circuit 11, and during a period inwhich the switching operation is stopped, the step-up circuit 10outputs, to the inverter circuit 11, DC power outputted from thephotovoltaic panel 2 and then inputted to the step-up circuit 10,without stepping up the DC input voltage value thereof.

A capacitor 19 (smoothing capacitor) for smoothing is connected betweenthe step-up circuit 10 and the inverter circuit 11.

The inverter circuit 11 includes switching elements Q1 to Q4 eachcomposed of a Field Effect Transistor (FET). The switching elements Q1to Q4 form a full-bridge circuit.

The switching elements Q1 to Q4 are connected to the control unit 12,and can be controlled by the control unit 12. The control unit 12performs PWM control of operations of the switching elements Q1 to Q4.Thereby, the inverter circuit 11 converts power given from the step-upcircuit 10 to AC power.

The inverter device 1 includes a filter circuit 21 between the invertercircuit 11 and the commercial power system 3.

The filter circuit 21 is composed of two AC reactors 22 and a capacitor23 (output smoothing capacitor) provided at a stage subsequent to the ACreactors 22. The filter circuit 21 has a function to remove ahigh-frequency component contained in AC power outputted from theinverter circuit 11. The AC power from which the high-frequencycomponent has been removed by the filter circuit 21 is given to thecommercial power system 3.

Thus, the step-up circuit 10 and the inverter circuit 11 form aconversion unit which converts DC power outputted from the photovoltaicpanel 2 to AC power and outputs the converted AC power to the commercialpower system 3 via the filter circuit 21.

A second current sensor 24 for detecting an inverter current detectionvalue Iinv (current flowing in the AC reactor 22) which is a currentvalue of output of the inverter circuit 11 is connected to the filtercircuit 21. A second voltage sensor 25 for detecting a voltage value(system voltage detection value Va) on the commercial power system 3side is connected between the filter circuit 21 and the commercial powersystem 3.

The second current sensor 24 and the second voltage sensor 25respectively output the detected inverter current detection value Iinvand the detected system voltage detection value Va (the voltage value ofthe AC system) to the control unit 12. Although the second currentsensor 24 is provided at a stage preceding the capacitor 23 as shown inFIG. 2, a third current sensor for detecting output current of theinverter device 1 may be added at a stage subsequent to the capacitor23.

The control unit 12 controls the step-up circuit 10 and the invertercircuit 11 based on the system voltage detection value Va, the invertercurrent detection value Iinv, the DC input voltage detection value Vg,and the step-up circuit current detection value Iin.

[1.2 Control Unit]

FIG. 3 is a block diagram of the control unit 12. As shown in FIG. 3,the control unit 12 functionally has a control processing unit 30, astep-up circuit control unit 32, an inverter circuit control unit 33,and an averaging processing unit 34.

Some or all of the functions of the control unit 12 may be configured asa hardware circuit, or may be realized by software (computer program)executed by a computer. Such software (computer program) for realizing afunction of the control unit 12 is stored in a storage device (notshown) of the computer.

The step-up circuit control unit 32 controls the switching element Qb ofthe step-up circuit 10 based on a target value and a detection valuegiven from the control processing unit 30, thereby causing the step-upcircuit 10 to output power having current corresponding to the targetvalue.

The inverter circuit control unit 33 controls the switching elements Q1to Q4 of the inverter circuit 11 based on a target value and a detectionvalue given from the control processing unit 30, thereby causing theinverter circuit 11 to output power having current corresponding to thetarget value.

The control processing unit 30 receives the DC input voltage detectionvalue Vg, the step-up circuit current detection value Iin, the systemvoltage detection value Va, and the inverter current detection valueIinv.

The control processing unit 30 calculates the input power Pin and anaverage value <Pin> thereof from the DC input voltage detection value Vgand the step-up circuit current detection value Iin.

The control processing unit 30 has a function to set a DC input currenttarget value Ig* (which will be described later) based on the inputpower average value <Pin> and perform MPPT control for the photovoltaicpanel 2, and to perform feedback control for the step-up circuit 10 andthe inverter circuit 11.

The DC input voltage detection value Vg and the step-up circuit currentdetection value Iin are given to the averaging processing unit 34 andthe control processing unit 30.

The averaging processing unit 34 has a function to sample, atpredetermined time intervals, the DC input voltage detection value Vgand the step-up circuit current detection value Iin given from the firstvoltage sensor 17 and the first current sensor 18, calculate theirrespective average values, and give the averaged DC input voltagedetection value Vg and the averaged step-up circuit current detectionvalue Iin to the control processing unit 30.

FIG. 4 is a graph showing an example of a simulation result of temporalchanges in the DC input voltage detection value Vg and the step-upcircuit current detection value Iin.

The DC input current detection value Ig is a current value detected onan input side relative to the capacitor 26.

As shown in FIG. 4, it is found that the DC input voltage detectionvalue Vg, the step-up circuit current detection value Iin, and the DCinput current detection value Ig vary in a half cycle of the systemvoltage.

The reason why the DC input voltage detection value Vg and the DC inputcurrent detection value Ig vary periodically as shown in FIG. 4 is asfollows. That is, the step-up circuit current detection value Iingreatly varies between almost OA and a peak value in a half cycle of theAC cycle in accordance with operations of the step-up circuit 10 and theinverter circuit 11. Therefore, the variation component cannot be fullyremoved by the capacitor 26, and the DC input current detection value Igis detected as pulsating current containing a component that varies in ahalf cycle of the AC cycle. On the other hand, output voltage of thephotovoltaic panel varies depending on output current.

Therefore, the cycle of the periodic variation occurring in the DC inputvoltage detection value Vg is half the cycle of AC power outputted fromthe inverter device 1.

The averaging processing unit 34 averages the DC input voltage detectionvalue Vg and the step-up circuit current detection value Iin in order tosuppress an influence of the above periodic variations.

FIG. 5 is a diagram showing a manner in which the averaging processingunit 34 averages the DC input voltage detection value Vg.

The averaging processing unit 34 samples the given DC input voltagedetection value Vg a plurality of times (at timings indicated by soliddots in FIG. 5) at predetermined time intervals Δt during a period Lfrom a timing t1 to a timing t2, and calculates an average value of theplurality of DC input voltage detection values Vg that have beenobtained.

Here, the averaging processing unit 34 sets the period L to half thelength of the cycle of the commercial power system 3. In addition, theaveraging processing unit 34 sets the time interval Δt to besufficiently shorter than half the length of the cycle of the commercialpower system 3.

Thus, the averaging processing unit 34 can accurately obtain the averagevalue of the DC input voltage detection value Vg which periodicallyvaries in synchronization with the cycle of the commercial power system3, using as short a sampling period as possible.

The time interval Δt of sampling may be set at, for example, 1/100 to1/1000 of the cycle of the commercial power system 3, or 20 microsecondsto 200 microseconds.

The averaging processing unit 34 may store the period L in advance, ormay acquire the system voltage detection value Va from the secondvoltage sensor 25 and set the period L based on the cycle of thecommercial power system 3.

Here, the period L is set to half the length of the cycle of thecommercial power system 3. The average value of the DC input voltagedetection value Vg can be accurately calculated at least if the period Lis set to half the cycle of the commercial power system 3. This isbecause the DC input voltage detection value Vg periodically varies in ahalf cycle of the commercial power system 3 in accordance withoperations of the step-up circuit 10 and the inverter circuit 11 asdescribed above.

Therefore, if it is required to set the period L to be longer, theperiod L may be set to an integer multiple of a half cycle of thecommercial power system 3, e.g., three or four times of a half cycle ofthe commercial power system 3. Thus, the voltage variation can begrasped on a cycle basis.

As described above, the step-up circuit current detection value Iin alsoperiodically varies in a half cycle of the commercial power system 3, asin the DC input voltage detection value Vg.

Therefore, the averaging processing unit 34 also calculates an averagevalue of the step-up circuit current detection value Iin by the samemethod as in the DC input voltage detection value Vg shown in FIG. 5.

The control processing unit 30 sequentially calculates an average valueof the DC input voltage detection value Vg and an average value of thestep-up circuit current detection value Iin per the period L.

The averaging processing unit 34 gives the calculated average value ofthe DC input voltage detection value Vg and the calculated average valueof the step-up circuit current detection value Iin to the controlprocessing unit 30.

In the present embodiment, as described above, the averaging processingunit 34 calculates an average value (DC input voltage average value<Vg>) of the DC input voltage detection value Vg and an average value(step-up circuit current average value <Iin>) of the step-up circuitcurrent detection value Iin, and using these values, the controlprocessing unit 30 controls the step-up circuit 10 and the invertercircuit 11 while performing MPPT control for the photovoltaic panel 2.Therefore, even if DC current from the photovoltaic panel 2 varies to beunstable, the control unit 12 can accurately obtain output of thephotovoltaic panel 2 as the DC input voltage average value <Vg> and thestep-up circuit current average value <Iin> in which a variationcomponent due to operation of the inverter device 1 has been removed. Asa result, it becomes possible to appropriately perform MPPT control andeffectively suppress reduction in power generation efficiency of thephotovoltaic panel 2.

As described above, in a case where voltage (DC input voltage detectionvalue Vg) or current (step-up circuit current detection value Iin) of DCpower outputted from the photovoltaic panel 2 varies due to operation ofthe inverter device 1, the cycle of the variation coincides with a halfcycle (a half cycle of the commercial power system 3) of AC poweroutputted from the inverter circuit 11.

In this regard, in the present embodiment, the DC input voltagedetection value Vg and the step-up circuit current detection value Iinare each sampled a plurality of times at the time intervals Δt which areshorter than a half cycle of the AC system, during the period L which isset to half the length of the cycle of the commercial power system 3,and the DC input voltage average value <Vg> and the step-up circuitcurrent average value <Iin> are calculated from a result of thesampling. Therefore, even if voltage and current of the DC current varyperiodically, the DC input voltage average value <Vg> and the step-upcircuit current average value <Iin> can be accurately calculated, withthe sampling period shortened as much as possible.

The control processing unit 30 sets the DC input current target valueIg* based on the above input power average value <Pin>, and calculatesrespective target values for the step-up circuit 10 and the invertercircuit 11 based on the set DC input current target value Ig* and theabove values.

The control processing unit 30 has a function of giving the calculatedtarget values to the step-up circuit control unit 32 and the invertercircuit control unit 33 and performing feedback control for the step-upcircuit 10 and the inverter circuit 11.

FIG. 6 is a control block diagram for explaining the feedback controlfor the step-up circuit 10 and the inverter circuit 11 by the controlprocessing unit 30.

The control processing unit 30 includes, as function sections forcontrolling the inverter circuit 11, a first calculation section 41, afirst adder 42, a compensator 43, and a second adder 44.

In addition, the control processing unit 30 includes, as a functionsection for controlling the step-up circuit 10, a second calculationsection 51, a third adder 52, a compensator 53, and a fourth adder 54.

FIG. 7 is a flowchart showing a control process for the step-up circuit10 and the inverter circuit 11. The function sections shown in FIG. 6control the step-up circuit 10 and the inverter circuit 11 by executingthe process shown in the flowchart in FIG. 7.

Hereinafter, the control process for the step-up circuit 10 and theinverter circuit 11 will be described with reference to FIG. 7.

First, the control processing unit 30 calculates the present input poweraverage value <Pin> (step S9), and compares the present input poweraverage value <Pin> with the input power average value <Pin> that hasbeen previously calculated, to set the DC input current target value Ig*(step S1). The input power average value <Pin> is calculated based onthe following expression (1).Input power average value <Pin>=<Iin×Vg>  (1)

In expression (1), Iin is the step-up circuit current detection value,and Vg is the DC input voltage detection value (DC input voltage value).For these values, the DC input voltage average value <Vg> and thestep-up circuit current average value <Iin> which are the valuesaveraged by the averaging processing unit 34 are used.

In each expression other than expression (1) and relevant to the controlshown below, instantaneous values which are not averaged are used forthe step-up circuit current detection value Iin and the DC input voltagedetection value Vg.

A notation “< >” indicates an average value of a value in the brackets.The same applies hereinafter.

The control processing unit 30 gives the set DC input current targetvalue Ig* to the first calculation section 41.

As well as the DC input current target value Ig*, the DC input voltagedetection value Vg and the system voltage detection value Va are givento the first calculation section 41.

The first calculation section 41 calculates an average value <Ia*> of anoutput current target value for the inverter device 1, based on thefollowing expression (2).Average value <Ia*> of output current target value=η<I _(g)*×Vg>/<Va>  (2)

where η is a constant representing the conversion efficiency of theinverter device 1.

Further, the first calculation section 41 calculates an output currenttarget value Ia* based on the following expression (3) (step S2).

Here, the first calculation section 41 calculates the output currenttarget value Ia* as a sine wave having the same phase as the systemvoltage detection value Va.Output current target value Ia*=(√2)×<Ia*>×sin ωt  (3)

As described above, the first calculation section 41 calculates theoutput current target value Ia* based on the input power average value<Pin> (an input power value of DC power) and the system voltagedetection value Va.

Next, the first calculation section 41 calculates an inverter currenttarget value Iinv* (a current target value for the inverter circuit)which is a current target value for controlling the inverter circuit 11,as shown by the following expression (4) (step S3).Inverter current target value Iinv*=Ia*+sCaVa  (4)

In expression (4), Ca is an electrostatic capacitance of the capacitor23 (output smoothing capacitor), and s is the Laplace operator.

The above expression (4) is represented as follows, using a derivativewith respect to time t.Iinv*=Ia*+Ca×(dVa/dt)  (4a)

If current flowing through the capacitor 23 is detected and the detectedcurrent is denoted by Ica, the following expression is obtained.Iinv*=Ia*+Ica  (4b)

In expressions (4), (4a), and (4b), the second term on the right-handside is a value added in consideration of current flowing through thecapacitor 23 of the filter circuit 21.

The output current target value Ia* is calculated as a sine wave havingthe same phase as the system voltage detection value Va, as shown by theabove expression (3). That is, the control processing unit 30 controlsthe inverter circuit 11 so that current Ia (output current) of AC poweroutputted from the inverter device 1 has the same phase as the systemvoltage (system voltage detection value Va).

After calculating the inverter current target value Iinv*, the firstcalculation section 41 gives the inverter current target value Iinv* tothe first adder 42.

The inverter circuit 11 is subjected to feedback control based on theinverter current target value Iinv*.

As well as the inverter current target value Iinv*, the present invertercurrent detection value Iinv is given to the first adder 42.

The first adder 42 calculates a difference between the inverter currenttarget value Iinv* and the present inverter current detection valueIinv, and gives a result of the calculation to the compensator 43.

When the difference is given, the compensator 43 performs calculationbased on a proportionality coefficient or the like, and further adds thesystem voltage Va by the second adder 44, thereby calculating aninverter voltage reference value Vinv# that allows the difference toconverge so that the inverter current detection value Iinv becomes theinverter current target value Iinv*. A control signal obtained bycomparing the inverter voltage reference value Vinv# with an outputvoltage target value Vo* for the DC/DC converter given from the firstcalculation section 41 is given to the inverter circuit control unit 33,thereby causing the inverter circuit 11 to output voltage according tothe inverter voltage reference value Vinv#.

The voltage outputted from the inverter circuit 11 is given to the ACreactor 22, and then fed back as a new inverter current detection valueIinv. Then, a difference between the inverter current target value Iinv*and the inverter current detection value Iinv is calculated again by thefirst adder 42, and the inverter circuit 11 is controlled based on thedifference as described above.

As described above, the inverter circuit 11 is subjected to feedbackcontrol based on the inverter current target value Iinv* and theinverter current detection value Iinv (step S4).

On the other hand, the inverter current target value Iinv* calculated bythe first calculation section 41, as well as the DC input voltagedetection value Vg and the system voltage detection value Va, is givento the second calculation section 51.

The second calculation section 51 calculates an inverter output voltagetarget value Vinv* (a voltage target value for the inverter circuit)based on the following expression (5) (step S5).Inverter output voltage target value Vinv*=Va+ZaIinv*  (5)

In expression (5), Za is an impedance of the AC reactor, and s in thefollowing is the Laplace operator.

The above expression (5) is represented as follows, using a derivativewith respect to time t.Vinv*=Va+RaIinv*+La×(dIinv*/dt)  (5a)

where Ra is a resistance of the AC reactor, La is an inductance of theAC reactor, and (Za=Ra+sLa) is satisfied.

The second term on the right-hand side in expression (5) and the secondterm and the third term on the right-hand side in expression (5a) arevalues added in consideration of voltage generated between both ends ofthe AC reactor 22.

Thus, in the present embodiment, the inverter output voltage targetvalue Vinv* is set based on the inverter current target value Iinv*which is the current target value for controlling the inverter circuit11 so that current of AC power outputted from the inverter device 1 hasthe same phase as the system voltage detection value Va.

As described above, the output target values (Iinv*, Vinv*) for theinverter circuit 11 which are target values on the AC side are set at abridge output end of the inverter circuit 11, i.e., a circuit connectionpoint P between the inverter circuit 11 and the filter circuit 21. Thus,the system interconnection is performed such that a point where thetarget values are set is moved to a stage preceding the original systeminterconnection point (a circuit connection point between the commercialpower system 3 and the filter circuit 21), whereby appropriate systeminterconnection is finally reached.

After calculating the inverter output voltage target value Vinv*, thesecond calculation section 51 compares the voltage Vg or preferably thefollowing DC voltage Vgf, as voltage V_(DC) on the DC power supply side,with an absolute value of the inverter output voltage target valueVinv*, and determines the greater one to be the step-up circuit voltagetarget value Vo* as shown by the following expression (6) (step S6). TheDC voltage Vgf is voltage calculated by considering voltage drop due toan impedance Z of the DC reactor 15 for Vg, and in a case where thestep-up circuit current is denoted by Iin, Vgf is represented asVgf=Vg−ZIin. Accordingly, Vo* can be represented as follows.Vo*=Max(Vg−ZIin,absolute value of Vinv*)  (6)

The above expression (6) is represented as follows, using a derivativewith respect to time t.Vo*=Max(Vg−(RIin+L(dIin/dt),absolute value of Vinv*)  (6a)

where R is a resistance of the DC reactor, L is an inductance of the DCreactor, and (Z=R+sL) is satisfied.

Further, the second calculation section 51 calculates the step-upcircuit current target value Iin* based on the following expression (7)(step S7).Step-up circuit current target valueIin*={(Iinv*×Vinv*)+(sCVo*)×Vo*}/(Vg−ZIin)  (7)

In expression (7), C is an electrostatic capacitance of the capacitor 19(smoothing capacitor), and s is the Laplace operator.

The above expression (7) is represented as follows, using a derivativewith respect to time t.Iin*={(Iinv*×Vinv*)+C×(dVo*/dt)×Vo*}/{Vg−(R+sL)Iin}  (7a)

If current flowing through the capacitor 19 is detected and the detectedcurrent is denoted by Ic, the following expression is obtained.Iin*={(Iinv*×Vinv*)+Ic×Vo*}/{Vg−ZIin}  (7b)

In expressions (7), (7a), and (7b), a term added to a product of theinverter current target value Iinv* and the inverter output voltagetarget value Vinv* is a value added in consideration of reactive powerpassing through the capacitor 19. That is, consideration of the reactivepower in addition to the power target value for the inverter circuit 11allows for more accurate calculation of the value of Iin*.

Further, if power loss P_(LOSS) of the inverter device 1 is measured inadvance, the above expression (7a) can be represented as follows.Iin*={(Iinv*×Vinv*)+C×(dVo*/dt)×Vo*+P _(LOSS) }/{Vg−ZIin}  (7c)

Similarly, the above expression (7b) can be represented as follows.Iin*={(Iinv*×Vinv*)+Ic×Vo*+P _(LOSS) }/{Vg−ZIin}  (7d)

In this case, consideration of the reactive power and the power lossP_(LOSS) in addition to the power target value of the inverter circuit11 allows for more strict calculation of the value of Iin*.

If the electrostatic capacitance C and the power loss P_(LOSS) of thecapacitor 19 are sufficiently smaller than (Iinv*×Vinv*), the followingexpression (8) is obtained. Iin* calculated by this expression (8) canbe used as Iin contained in the right-hand sides of expressions (6),(6a), (7), (7a), (7b), (7c), and (7d).Step-up circuit current target value Iin*=(Iinv*×Vinv*)/Vg  (8)

After calculating the step-up circuit current target value Iin*, thesecond calculation section 51 gives the step-up circuit current targetvalue Iin* to the third adder 52.

The step-up circuit 10 is subjected to feedback control based on thestep-up circuit current target value Iin*.

As well as the step-up circuit current target value Iin*, the presentstep-up circuit current detection value Iin is given to the third adder52.

The third adder 52 calculates a difference between the step-up circuitcurrent target value Iin* and the present step-up circuit currentdetection value Iin, and gives a result of the calculation to thecompensator 53.

When the above difference is given, the compensator 53 performscalculation based on a proportionality coefficient or the like, andfurther subtracts the resultant value from the DC input voltagedetection value Vg by the fourth adder 54, thereby calculating a step-upcircuit voltage reference value Vbc# that allows the difference toconverge so that the step-up circuit current detection value Iin becomesthe step-up circuit current target value Iin*. A control signal obtainedby comparing the step-up circuit voltage reference value Vbc# with theoutput voltage target value Vo* for the DC/DC converter given from thefirst calculation section 41 is given to the step-up circuit controlunit 32, thereby causing the step-up circuit 10 to output voltageaccording to the step-up circuit voltage reference value Vbc#.

The power outputted from the step-up circuit 10 is given to the DCreactor 15, and then fed back as a new step-up circuit current detectionvalue Iin. Then, a difference between the step-up circuit current targetvalue Iin* and the step-up circuit current detection value Iin iscalculated again by the third adder 52, and the step-up circuit 10 iscontrolled based on the difference as described above.

As described above, the step-up circuit 10 is subjected to feedbackcontrol based on the step-up circuit current target value Iin* and thestep-up circuit current detection value Iin (step S8).

After the above step S8, the control processing unit 30 calculates thepresent input power average value <Pin> based on the above expression(1) (step S9).

Based on comparison with the input power average value <Pin> that hasbeen previously calculated, the control processing unit 30 sets the DCinput current target value Ig* so that the input power average value<Pin> becomes a maximum value (follows the maximum power point).

Thus, the control processing unit 30 controls the step-up circuit 10 andthe inverter circuit 11 while performing MPPT control for thephotovoltaic panel 2.

As described above, the control processing unit 30 performs feedbackcontrol for the inverter circuit 11 and the step-up circuit 10 by thecurrent target values.

FIG. 8 is graphs in which (a) shows an example of a simulation result ofthe step-up circuit current target value Iin* calculated in the abovefeedback control by the control processing unit 30, and the step-upcircuit current detection value Iin obtained when control is performedin accordance with the step-up circuit current target value Iin*, and(b) shows an example of a simulation result of the step-up circuitvoltage target value Vo* calculated in the above feedback control by thecontrol processing unit 30, and a step-up circuit voltage detectionvalue Vo obtained when control is performed in accordance with thestep-up circuit voltage target value Vo*.

As shown in (a) of FIG. 8, it is found that the step-up circuit currentdetection value Iin is controlled along the step-up circuit currenttarget value Iin* by the control processing unit 30.

As shown in (b) of FIG. 8, since the step-up circuit voltage targetvalue Vo* is calculated by the above expression (6), the step-up circuitvoltage target value Vo* varies so as to follow an absolute value of theinverter output voltage target value Vinv* during a period in which theabsolute value of the inverter output voltage target value Vinv* isgenerally equal to or greater than the DC input voltage detection valueVg, and follow the DC input voltage detection value Vg during the otherperiod.

It is found that the step-up circuit voltage detection value Vo iscontrolled along the step-up circuit voltage target value Vo* by thecontrol processing unit 30.

FIG. 9 is a diagram showing an example of the inverter output voltagetarget value Vinv*. In FIG. 9, the vertical axis indicates voltage andthe horizontal axis indicates time. A broken line indicates a voltagewaveform of the commercial power system 3, and a solid line indicates awaveform of the inverter output voltage target value Vinv*.

The inverter circuit 11 outputs power, using the inverter output voltagetarget value Vinv* shown in FIG. 9 as a voltage target value, throughthe control according to the flowchart in FIG. 7.

Therefore, the inverter circuit 11 outputs power having voltageaccording to the waveform of the inverter output voltage target valueVinv* shown in FIG. 9.

As shown in FIG. 9, the two waveforms have almost the same voltage valueand the same frequency, but the phase of the inverter output voltagetarget value Vinv* leads the phase of voltage of the commercial powersystem 3 by several degrees.

The control processing unit 30 of the present embodiment causes thephase of the inverter output voltage target value Vinv* to lead thephase of voltage of the commercial power system 3 by about three degreeswhile executing the feedback control for the step-up circuit 10 and theinverter circuit 11, as described above.

The degree of angle by which the phase of the inverter output voltagetarget value Vinv* is caused to lead the phase of voltage of thecommercial power system 3 may be several degrees, and as describedlater, the degree of angle is set within such a range that the phase ofa voltage waveform of a difference from a voltage waveform of thecommercial power system 3 leads the phase of the voltage waveform of thecommercial power system 3 by 90 degrees. For example, the degree of thephase leading angle is set to be greater than 0 degrees and smaller than10 degrees.

The degree of the phase leading angle is determined by the systemvoltage detection value Va, the inductance La of the AC reactor 22, andthe inverter current target value Iinv* as shown by the above expression(5). Of these values, the system voltage detection value Va and theinductance La of the AC reactor 22 are fixed values that are not controltargets. Therefore, the degree of the phase leading angle is determinedby the inverter current target value Iinv*.

The inverter current target value Iinv* is determined by the outputcurrent target value Ia* as shown by the above expression (4). As theoutput current target value Ia* increases, a phase leading component ofthe inverter current target value Iinv* increases, and a leading angle(phase leading angle) of the inverter output voltage target value Vinv*increases.

Since the output current target value Ia* is calculated by the aboveexpression (2), the phase leading angle is adjusted by the DC inputcurrent target value Ig*.

[1.3 Control for Step-Up Circuit and Inverter Circuit]

The step-up circuit control unit 32 controls the switching element Qb ofthe step-up circuit 10. The inverter circuit control unit 33 controlsthe switching elements Q1 to Q4 of the inverter circuit 11.

The step-up circuit control unit 32 and the inverter circuit controlunit 33 respectively generate a step-up circuit carrier wave and aninverter circuit carrier wave, and respectively modulate these carrierwaves with the step-up circuit voltage reference value Vbc# and theinverter voltage reference value Vinv# which are target values givenfrom the control processing unit 30, to generate drive waveforms fordriving each switching element.

The step-up circuit control unit 32 and the inverter circuit controlunit 33 control each switching element based on the drive waveforms,thereby causing the step-up circuit 10 and the inverter circuit 11 tooutput AC powers having current waveforms approximate to the step-upcircuit current target value Iin* and the inverter current target valueIinv*, respectively.

In FIG. 10, (a) is a graph showing comparison between the step-upcircuit carrier wave and a waveform of the step-up circuit voltagereference value Vbc#. In (a) of FIG. 10, the vertical axis indicatesvoltage and the horizontal axis indicates time. In (a) of FIG. 10, forfacilitating the understanding, the wavelength of the step-up circuitcarrier wave is elongated as compared to the actual wavelength.

The step-up circuit carrier wave generated by the step-up circuitcontrol unit 32 is a triangle wave having a minimum value of “0”, andhas an amplitude A1 set at the step-up circuit voltage target value Vo*given from the control processing unit 30.

The frequency of the step-up circuit carrier wave is set by the step-upcircuit control unit 32 in accordance with a control command from thecontrol processing unit 30, so as to realize a predetermined duty ratio.

As described above, the step-up circuit voltage target value Vo* variesso as to follow an absolute value of the inverter output voltage targetvalue Vinv* during a period W1 in which the absolute value of theinverter output voltage target value Vinv* is generally equal to orgreater than the DC input voltage detection value Vg, and follow the DCinput voltage detection value Vg during the other period. Therefore, theamplitude A1 of the step-up circuit carrier wave also varies inaccordance with the step-up circuit voltage target value Vo*.

In the present embodiment, the DC input voltage detection value Vg is250 volts, and the amplitude of voltage of the commercial power system 3is 288 volts.

A waveform (hereinafter, may be referred to as a step-up circuitreference wave Vbc#) of the step-up circuit voltage reference value Vbc#corresponds to a value calculated based on the step-up circuit currenttarget value Iin* by the control processing unit 30, and has a positivevalue during the period W1 in which the absolute value of the inverteroutput voltage target value Vinv* is greater than the DC input voltagedetection value Vg. During the period W1, the step-up circuit referencewave Vbc# has a waveform approximate to the shape of a waveform createdby the step-up circuit voltage target value Vo*, and crosses the step-upcircuit carrier wave.

The step-up circuit control unit 32 compares the step-up circuit carrierwave with the step-up circuit reference wave Vbc#, and generates a drivewaveform for driving the switching element Qb so as to be turned onduring a period in which the step-up circuit reference wave Vbc# whichis a target value for voltage between both ends of the DC reactor 15 isequal to or greater than the step-up circuit carrier wave, and to beturned off during a period in which the step-up circuit reference waveVbc# is equal to or smaller than the carrier wave.

In FIG. 10, (b) shows the drive waveform for driving the switchingelement Qb, generated by the step-up circuit control unit 32. In (b) ofFIG. 10, the vertical axis indicates voltage and the horizontal axisindicates time. The horizontal axis in (b) of FIG. 10 coincides withthat in (a) of FIG. 10.

The drive waveform indicates switching operation of the switchingelement Qb. When the drive waveform is given to the switching elementQb, the switching element Qb is caused to perform switching operation inaccordance with the drive waveform. The drive waveform forms a controlcommand to turn off the switching element when the voltage is 0 voltsand turn on the switching element when the voltage is a plus voltage.

The step-up circuit control unit 32 generates the drive waveform so thatthe switching operation is performed during the period W1 in which theabsolute value of the inverter output voltage target value Vinv* isequal to or greater than the DC input voltage detection value Vg.Therefore, in a range in which the absolute value is equal to or smallerthan the DC input voltage detection value Vg, the switching element Qbis controlled to stop the switching operation.

Each pulse width is determined by an intercept of the step-up circuitcarrier wave which is a triangle wave. Therefore, the pulse width isgreater at a part where voltage is higher.

As described above, the step-up circuit control unit 32 modulates thestep-up circuit carrier wave with the step-up circuit reference waveVbc#, to generate the drive waveform representing pulse widths forswitching. The step-up circuit control unit 32 performs PWM control forthe switching element Qb of the step-up circuit 10, based on thegenerated drive waveform.

In the case where a switching element Qbu that conducts current in aforward direction of the diode 16 is provided in parallel with the diode16, a drive waveform inverted from the drive waveform for the switchingelement Qb is used for the switching element Qbu. In order to preventthe switching element Qb and the switching element Qbu from conductingcurrents at the same time, a dead time of about 1 microsecond isprovided at a part where a drive pulse for the switching element Qbushifts from OFF to ON.

In FIG. 11, (a) is a graph showing comparison between the invertercircuit carrier wave and a waveform of the inverter voltage referencevalue Vinv#. In (a) of FIG. 11, the vertical axis indicates voltage andthe horizontal axis indicates time. Also in (a) of FIG. 11, forfacilitating the understanding, the wavelength of the inverter circuitcarrier wave is elongated as compared to the actual wavelength.

The inverter circuit carrier wave generated by the inverter circuitcontrol unit 33 is a triangle wave having an amplitude center at 0volts, and a one-side amplitude thereof is set at the step-up circuitvoltage target value Vo* (a voltage target value for the capacitor 23).Therefore, the inverter circuit carrier wave has a period in which anamplitude A2 thereof is twice (500 volts) as great as the DC inputvoltage detection value Vg and a period in which the amplitude A2 istwice (576 volts at maximum) as great as voltage of the commercial powersystem 3.

The frequency thereof is set by the inverter circuit control unit 33 inaccordance with a control command from the control processing unit 30,or the like, so as to realize a predetermined duty ratio.

As described above, the step-up circuit voltage target value Vo* variesto follow an absolute value of the inverter output voltage target valueVinv* during the period W1 in which the absolute value of the inverteroutput voltage target value Vinv* is generally equal to or greater thanthe DC input voltage detection value Vg, and follow the DC input voltagedetection value Vg during the other period, i.e., a period W2.Therefore, the amplitude A2 of the inverter circuit carrier wave alsovaries in accordance with the step-up circuit voltage target value Vo*.

A waveform (hereinafter, may be referred to as an inverter circuitreference wave Vinv#) of the inverter voltage reference value Vinv#corresponds to a value calculated based on the inverter current targetvalue Iinv* by the control processing unit 30, and is set to havegenerally the same amplitude as the voltage amplitude (288 volts) of thecommercial power system 3. Therefore, the inverter circuit referencewave Vinv# crosses the inverter circuit carrier wave in a range wherethe voltage value is between −Vg and +Vg.

The inverter circuit control unit 33 compares the inverter circuitcarrier wave with the inverter circuit reference wave Vinv#, andgenerates drive waveforms for driving the switching elements Q1 to Q4 soas to be turned on during a period in which the inverter circuitreference wave Vinv# which is a voltage target value is equal to orgreater than the inverter circuit carrier wave, and to be turned offduring a period in which the inverter circuit reference wave Vinv# isequal to or smaller than the carrier wave.

In FIG. 11, (b) shows the drive waveform for driving the switchingelement Q1, generated by the inverter circuit control unit 33. In (b) ofFIG. 11, the vertical axis indicates voltage and the horizontal axisindicates time. The horizontal axis in (b) of FIG. 11 coincides withthat in (a) of FIG. 11.

The inverter circuit control unit 33 generates the drive waveform sothat the switching operation is performed in the range W2 in whichvoltage of the inverter circuit reference wave Vinv# is between −Vg and+Vg. Therefore, in the other range, the switching element Q1 iscontrolled to stop the switching operation.

In FIG. 11, (c) shows the drive waveform for driving the switchingelement Q3, generated by the inverter circuit control unit 33. In (c) ofFIG. 11, the vertical axis indicates voltage and the horizontal axisindicates time.

The inverter circuit control unit 33 compares the carrier wave with awaveform indicated by a broken line in (a) of FIG. 11, which is invertedfrom the inverter circuit reference wave Vinv#, to generate the drivewaveform for the switching element Q3.

Also in this case, the inverter circuit control unit 33 generates thedrive waveform so that the switching operation is performed in the rangeW2 in which voltage of (a waveform inverted from) the inverter circuitreference wave Vinv# is between −Vg and +Vg. Therefore, in the otherrange, the switching element Q3 is controlled to stop the switchingoperation.

The inverter circuit control unit 33 generates, as the drive waveformfor the switching element Q2, a waveform inverted from the drivewaveform for the switching element Q1, and generates, as the drivewaveform for the switching element Q4, a waveform inverted from thedrive waveform for the switching element Q3.

As described above, the inverter circuit control unit 33 modulates theinverter circuit carrier wave with the inverter circuit reference waveVinv#, to generate the drive waveforms representing pulse widths forswitching. The inverter circuit control unit 33 performs PWM control forthe switching elements Q1 to Q4 of the inverter circuit 11, based on thegenerated drive waveforms.

The step-up circuit control unit 32 of the present embodiment causes thestep-up circuit 10 to output power so that current flowing in the DCreactor 15 coincides with the step-up circuit current target value Iin*.As a result, the step-up circuit 10 is caused to perform switchingoperation during the period W1 (FIG. 10) in which an absolute value ofthe inverter output voltage target value Vinv* is generally equal to orgreater than the DC input voltage detection value Vg. The step-upcircuit 10 outputs power having voltage equal to or greater than the DCinput voltage detection value Vg and approximate to the absolute valueof the inverter output voltage target value Vinv*, during the period W1.On the other hand, during the period in which the absolute value of theinverter output voltage target value Vinv* is generally equal to orsmaller than the DC input voltage detection value Vg, the step-upcircuit control unit 32 stops the switching operation of the step-upcircuit 10. Therefore, during the period in which the absolute value isequal to or smaller than the DC input voltage detection value Vg, thestep-up circuit 10 outputs, to the inverter circuit 11, DC poweroutputted from the photovoltaic panel 2 without stepping up the DC inputvoltage value thereof.

The inverter circuit control unit 33 of the present embodiment causesthe inverter circuit 11 to output power so that current flowing in theAC reactor 22 coincides with the inverter current target value Iinv*. Asa result, the inverter circuit 11 is caused to perform switchingoperation during the period W2 (FIG. 11) in which the inverter outputvoltage target value Vinv* is generally between −Vg and +Vg. That is,the inverter circuit 11 is caused to perform switching operation duringa period in which an absolute value of the inverter output voltagetarget value Vinv* is equal to or smaller than the DC input voltagedetection value Vg.

Therefore, while switching operation of the step-up circuit 10 isstopped, the inverter circuit 11 performs switching operation to outputAC power approximate to the inverter output voltage target value Vinv*.

Since the inverter circuit reference wave Vinv# and the inverter outputvoltage target value Vinv* are approximate to each other, they overlapeach other in (a) of FIG. 11.

On the other hand, in the period other than the period W2 in whichvoltage of the inverter output voltage target value Vinv* is generallybetween −Vg and +Vg, the inverter circuit control unit 33 stops theswitching operation of the inverter circuit 11. During this period,power stepped up by the step-up circuit 10 is given to the invertercircuit 11. Therefore, the inverter circuit 11 whose switching operationis stopped outputs the power given from the step-up circuit 10, withoutstepping down the voltage thereof.

That is, the inverter device 1 of the present embodiment causes thestep-up circuit 10 and the inverter circuit 11 to perform switchingoperations so as to be alternately switched therebetween, andsuperimposes their respective output powers on each other, therebyoutputting AC power having a voltage waveform approximate to theinverter output voltage target value Vinv*.

Thus, in the present embodiment, control is performed so that thestep-up circuit 10 is operated in a case of outputting voltagecorresponding to the part where the absolute value of the inverteroutput voltage target value Vinv* is higher than the DC input voltagedetection value Vg, and the inverter circuit 11 is operated in a case ofoutputting voltage corresponding to the part where the absolute value ofthe inverter output voltage target value Vinv* is lower than the DCinput voltage detection value Vg. Therefore, since the inverter circuit11 does not step down the power that has been stepped up by the step-upcircuit 10, a potential difference in stepping down of the voltage canbe reduced, whereby loss due to switching of the step-up circuit isreduced and AC power can be outputted with increased efficiency.

Further, since both the step-up circuit 10 and the inverter circuit 11operate based on the inverter output voltage target value Vinv* set bythe control unit 12, occurrence of deviation or distortion between powerof the step-up circuit and power of the inverter circuit which areoutputted so as to be alternately switched can be suppressed.

FIG. 12 is a diagram showing examples of the reference waves and thedrive waveforms for the switching elements, and an example of a currentwaveform of AC power outputted from the inverter device 1.

FIG. 12 shows graphs of, from the uppermost side, the reference waveVinv# and the carrier wave for the inverter circuit, the drive waveformfor the switching element Q1, the reference wave Vbc# and the carrierwave for the step-up circuit, the drive waveform for the switchingelement Qb, and the target value and an actual measured value of acurrent waveform of AC power outputted from the inverter device 1. Thehorizontal axes of these graphs indicate time, and coincide with eachother.

As shown in FIG. 12, it is found that output current is controlled sothat an actual measured value Ia thereof coincides with a target valueIa*.

In addition, it is found that the period in which the switching elementQb of the step-up circuit 10 performs switching operation and the periodin which the switching elements Q1 to Q4 of the inverter circuit 11perform switching operations are controlled so as to be generallyalternately switched therebetween.

In the present embodiment, as shown in (a) of FIG. 8, the step-upcircuit is controlled so that current flowing in the DC reactor 15coincides with the current target value Iin* calculated based on theabove expression (7). As a result, voltages of the step-up circuit andthe inverter circuit have waveforms as shown in (b) of FIG. 8, and itbecomes possible to perform such an operation that high-frequencyswitching operations of the step-up circuit 10 and the inverter circuit11 have respective stop periods and the switching operations areperformed generally alternately.

Ideally, it is preferable that the step-up circuit 10 and the invertercircuit 11 “alternately” perform high-frequency switching so that theirrespective periods of high-frequency switching do not overlap eachother. However, in practice, even if both periods slightly overlap eachother, as long as a stop period is provided for each of the step-upcircuit 10 and the inverter circuit 11, the loss can be reduced, leadingto enhancement in the efficiency.

1.4 Current Phase of Outputted AC Power

The step-up circuit 10 and the inverter circuit 11 of the presentembodiment output AC power having a voltage waveform approximate to theinverter output voltage target value Vinv*, to the filter circuit 21connected at the subsequent stage, through the control by the controlunit 12. The inverter device 1 outputs AC power to the commercial powersystem 3 via the filter circuit 21.

Here, the inverter output voltage target value Vinv* is generated by thecontrol processing unit 30 so as to have a voltage phase leading thevoltage phase of the commercial power system 3 by several degrees asdescribed above.

Therefore, AC voltage outputted by the step-up circuit 10 and theinverter circuit 11 also has a voltage phase leading the voltage phaseof the commercial power system 3 by several degrees.

As a result, the AC voltage from the step-up circuit 10 and the invertercircuit 11 is applied to one end of the AC reactor 22 (FIG. 2) of thefilter circuit 21, and voltage of the commercial power system 3 isapplied to the other end. Thus, voltages having phases shifted from eachother by several degrees are applied to the respective ends of the ACreactor 22.

In FIG. 13, (a) is a graph showing voltage waveforms of AC voltageoutputted from the inverter circuit 11, the commercial power system 3,and voltage between both ends of the AC reactor 22. In (a) of FIG. 13,the vertical axis indicates voltage and the horizontal axis indicatestime.

As shown in (a) of FIG. 13, when voltages having phases shifted fromeach other by several degrees are applied to the respective ends of theAC reactor 22, the voltage between both ends of the AC reactor 22 isequal to a difference between the voltages applied to the respectiveends of the AC reactor 22 and having phases shifted from each other byseveral degrees.

Therefore, as shown in (a) of FIG. 13, the phase of voltage between bothends of the AC reactor 22 leads the phase of voltage of the commercialpower system 3 by 90 degrees.

In FIG. 13, (b) is a graph showing a waveform of current flowing in theAC reactor 22. In (b) of FIG. 13, the vertical axis indicates currentand the horizontal axis indicates time. The horizontal axis in (b) ofFIG. 13 coincides with that in (a) of FIG. 13.

The current phase of the AC reactor 22 lags the voltage phase thereof by90 degrees. Therefore, as shown in (b) of FIG. 13, the current phase ofAC power outputted through the AC reactor 22 is synchronized with thecurrent phase of the commercial power system 3.

Therefore, although the phase of voltage outputted from the invertercircuit 11 leads the phase of the commercial power system 3 by severaldegrees, the phase of current outputted from the inverter circuit 11coincides with the phase of current of the commercial power system 3.

Therefore, as shown in the lowermost graph in FIG. 12, the phase of acurrent waveform outputted from the inverter device 1 coincides with thevoltage phase of the commercial power system 3.

As a result, AC current in phase with voltage of the commercial powersystem 3 can be outputted, whereby reduction in a power factor of the ACpower can be suppressed.

In FIG. 22, (a) is an example of the AC output waveform of the inverterdevice 1 according to the above embodiment. The step-up circuit currenttarget value Iin* in this case is given by expression (7), for example.

Thus, AC output current having a sine waveform synchronized with thesystem voltage is obtained. In this case, the power factor is 0.997 andan overall current distortion rate is 4.6%, and thus they are adequatefor respective criterion values in system interconnection which aregenerally set at 0.95 or higher and at 5% or lower, respectively.Besides, a second-order distortion rate is 2.6% (adequate for 3% orlower), a third-order distortion rate is 2.9% (adequate for 3% orlower), and a fifth-order distortion rate is 0.3% (adequate for 3% orlower).

On the other hand, (b) of FIG. 22 is an example of an AC output waveformobtained when the inverter device 1 is controlled in accordance with astep-up circuit current target value prescribed by the followingexpression (9) described in the aforementioned Patent Literature 2.Iin*=Ia*×Va/Vg  (9)

In this case, the AC output current has a waveform the peak of which isclearly distorted. The power factor is 0.947 (inadequate for 0.95 orhigher) and the overall current distortion rate is 8.3% (inadequate for5% or lower), and thus both of them are not adequate for the abovecriterion values in system interconnection. Besides, a second-orderdistortion rate is 3.5% (inadequate for 3% or lower), a third-orderdistortion rate is 4.3% (inadequate for 3% or lower), and a fifth-orderdistortion rate is 4.6% (inadequate for 3% or lower).

2. Second Embodiment

FIG. 14 is an example of a circuit diagram of an inverter device 1according to the second embodiment.

A difference between the present embodiment and the first embodiment isthat IGBTs are used as the switching elements Q1 to Q4 of the invertercircuit 11. The other configuration is the same as in the firstembodiment.

In the present embodiment, the inverter circuit control unit 33 uses acarrier wave different from the inverter circuit carrier wave used inthe above first embodiment.

FIG. 15 is a graph showing comparison between an inverter circuitcarrier wave in the second embodiment and a reference wave. In FIG. 15,the vertical axis indicates voltage and the horizontal axis indicatestime.

The reference wave and a step-up circuit carrier wave are the same asthose in the first embodiment.

On the other hand, the inverter circuit carrier wave of the presentembodiment is a triangle wave having a lower limit value set at 0 voltsand an upper limit value set at the step-up circuit voltage target valueVo*.

In this case, the inverter circuit control unit 33 generates a drivewaveform for the switching element Q1 based on comparison between theinverter circuit reference wave Vinv# and the inverter circuit carrierwave, and generates a drive waveform for the switching element Q3 basedon comparison between a wave inverted from the inverter circuitreference wave Vinv#, and the inverter circuit carrier wave.

Also in the present embodiment, the inverter circuit control unit 33(step-up circuit control unit 32) compares the inverter circuit carrierwave (step-up circuit carrier wave) with the inverter circuit referencewave Vinv#, and generates drive waveforms for driving the switchingelements so as to be turned on during a period in which the invertercircuit reference wave Vinv# (or a wave inverted therefrom) which is avoltage target value is equal to or greater than the inverter circuitcarrier wave (step-up circuit carrier wave), and to be turned off duringa period in which the inverter circuit reference wave Vinv# (or a waveinverted therefrom) is equal to or smaller than the carrier wave.

FIG. 16 is a diagram showing examples of the drive waveforms for theswitching elements Qb and Q1 to Q4, and an example of a current waveformof AC power outputted from the inverter device 1, in the secondembodiment.

FIG. 16 shows graphs of, from the uppermost side, the drive waveform forthe switching element Q1, the drive waveform for the switching elementQ4, the drive waveform for the switching element Q3, the drive waveformfor the switching element Q2, the drive waveform for the switchingelement Qb, and a current waveform of AC power outputted from theinverter device 1. The horizontal axes of these graphs indicate time,and coincide with each other.

In the present embodiment, the switching element Q1 and the switchingelement Q3 are controlled to perform switching in a range where voltageof the inverter circuit reference wave Vinv# is between −Vg and +Vg.

Also in the present embodiment, as shown in FIG. 16, it is found thatthe period in which the switching element Qb of the step-up circuit 10performs switching operation and the period in which the switchingelements Q1 to Q4 of the inverter circuit 11 perform switchingoperations are controlled so as to be alternately switched therebetween.

The phase of a current waveform of AC power outputted from the inverterdevice 1 of the present embodiment coincides with the voltage phase ofthe commercial power system 3, as shown in FIG. 16. Therefore, as in theabove first embodiment, AC power having the same current phase as thecommercial power system 3 can be outputted, whereby reduction in a powerfactor of the AC power can be suppressed.

3. Third Embodiment

FIG. 17 shows an example of a circuit diagram of an inverter device 1according to the third embodiment.

A difference between the present embodiment and the first embodiment isthat a third voltage sensor 27 for detecting intermediate voltagebetween the step-up circuit 10 and the inverter circuit 11 is provided.The other configuration is the same as in the first embodiment.

In the above first embodiment, the step-up circuit voltage target valueVo* (a target value of the intermediate voltage) is used as theamplitude of the carrier wave, but in the present embodiment, thevoltage detection value Vo detected by the third voltage sensor 27 isused as the amplitude of the carrier wave.

FIG. 18 is a diagram showing examples of the reference waves and thedrive waveforms for the switching elements, and an example of a currentwaveform of AC power outputted from the inverter device 1, in the thirdembodiment.

FIG. 18 shows graphs of, from the uppermost side, the reference waveVinv# and the carrier wave for the inverter circuit, the drive waveformfor the switching element Q1, the reference wave Vbc# and the carrierwave for the step-up circuit, the drive waveform for the switchingelement Qb, and the target value Ia* and the actual measured value Ia ofa current waveform of AC power outputted from the inverter device 1. Thehorizontal axes of these graphs indicate time, and coincide with eachother.

As shown in FIG. 18, also in the present embodiment, it is found thatoutput current is controlled so that the actual measured value Iathereof coincides with the target value Ia*.

In addition, it is found that the period in which the switching elementQb of the step-up circuit 10 performs switching operation and the periodin which the switching element Q1 of the inverter circuit 11 performsswitching operation are controlled so as to be generally alternatelyswitched therebetween.

As in the present embodiment, if the voltage detection value Vo is usedas the amplitude of the carrier wave, response to variation in voltageof the photovoltaic panel 2 or the commercial power system 3 becomesfast, and output current of the inverter device 1 can be stabilized.

4. Supplementary Note

It has been verified that the same result as in each simulation in theabove embodiments can be obtained using an actual machine.

<<AC-to-DC Conversion Device>>

[Overall Configuration]

Next, an embodiment of a conversion device 1R that performs powerconversion from AC to DC will be described.

FIG. 19 is a block diagram showing an example of a power storage systemincluding the conversion device 1R. In FIG. 19, a storage battery 2 isconnected to an output end of the conversion device 1R, and thecommercial power system 3 (AC system) is connected to an input end ofthe conversion device 1R. The power storage system is capable ofconverting power provided from the commercial power system 3, from AC toDC, and storing the converted power in the storage battery 2.

The conversion device 1R includes: an AC/DC converter 11 u whichconverts AC power received from the commercial power system 3 to DCpower; a step-down circuit (DC/DC converter) 10 d which steps downoutput voltage of the AC/DC converter 11 u; and the control unit 12which controls operations of these circuits 10 d and 11 u. As is obviousfrom comparison with FIG. 1, the direction of energy flow is reversed.

FIG. 20 is an example of a circuit diagram of the conversion device 1R.As a difference from FIG. 2, FIG. 14, and FIG. 17 (hereinafter, FIG. 2,etc.), first, the photovoltaic panel 2 in FIG. 2, etc. is replaced witha storage battery 2B. In addition, in the conversion device 1R, thestep-up circuit 10 in FIG. 2, etc. is replaced with the step-downcircuit 10 d, and the inverter circuit 11 in FIG. 2, etc. is replacedwith the AC/DC converter 11 u which is capable of also step-up operationin cooperation with the AC reactor 22 though the components thereof arethe same.

The step-down circuit 10 d is provided with a switching element Qb2 inparallel with the same diode 16 as in FIG. 2, etc. As the switchingelement Qb2, the shown IGBT or FET can be used, for example.

The other configuration of the conversion device 1R is basically thesame as that of the inverter device 1 in FIG. 2, etc. Therefore, theconversion device 1R has a bidirectional property, and is capable ofperforming the same operation as in the inverter device 1 in FIG. 2,etc. when a photovoltaic panel is connected. In addition, the conversiondevice 1R is also capable of performing autonomous operation byconverting DC power of the storage battery 2B to AC power.

In the case where the conversion device 1R operates as an inverterdevice, the switching element Qb2 is controlled by the control unit 12so as to be OFF constantly (in a case of IGBT) or so as to be turned onalternately with the switching element Qb (in a case of FET). Inaddition, the step-down circuit 10 d serves as a step-up circuit, andthe AC/DC converter 11 u serves as an inverter circuit.

In the case of charging the storage battery 2B based on AC power of thecommercial AC system 3, the control unit 12 can perform synchronousrectification by controlling operations of the switching elements Q1 toQ4. In addition, by performing PWM control under the presence of the ACreactor 22, the control unit 12 can perform rectification whileperforming step-up operation. Thus, the AC/DC converter 11 u converts ACpower given from the commercial AC system 3 to DC power.

The step-down circuit 10 d forms a step-down chopper circuit. Theswitching elements Qb and Qb2 are controlled by the control unit 12.

The switching operation of the step-down circuit 10 d is controlled sothat a period in which the step-down circuit 10 d performs switchingoperation and a period in which the AC/DC converter 11 u performsswitching operation are alternately switched. Therefore, during a periodin which the step-down circuit 10 d performs switching operation, thestep-down circuit 10 d outputs stepped-down voltage to the storagebattery 2B, and during a period in which the step-down circuit 10 dstops the switching operation (the switching element Qb is OFF and theswitching element Qb2 is ON), the step-down circuit 10 d gives DCvoltage outputted from the AC/DC converter 11 u and inputted to thestep-down circuit 10 d, to the storage battery 2 via the DC reactor 15.

[Summary of Voltage Waveform]

FIG. 21 is a voltage waveform diagram conceptually showing operation ofthe conversion device 1R.

In FIG. 21, (a) shows an example of an absolute value of an AC inputvoltage target value Vinv* for the AC/DC converter 11 u. This generallycorresponds to a full-wave-rectified waveform based on the commercialAC. A two-dot dashed line indicates DC voltage Vg for charging. As shownin (b) of FIG. 21, during periods (from t0 to t1, from t2 to t3, fromt4) in which the DC voltage Vg is higher than the absolute value of theAC input voltage target value Vinv*, the AC/DC converter 11 u performsswitching operation and performs step-up operation in cooperation withthe AC reactor 22.

Meanwhile, during these periods (from t0 to t1, from t2 to t3, from t4),in the step-down circuit 10 d, the switching element Qb is OFF and theswitching element Qb2 is ON, and the step-down circuit 10 d stopsstep-down operation. It is noted that thin stripes shown in (b) of FIG.21 are actually a PWM pulse train, and the duty thereof varies inaccordance with the absolute value of the AC input voltage target valueVinv*. Therefore, if the voltage in this state is applied to the DC/DCconverter, input voltage of the DC/DC converter, i.e., voltage of thecapacitor 19 has a waveform as shown in (c) of FIG. 21.

On the other hand, during periods (from t1 to t2, from t3 to t4) inwhich the DC voltage Vg is lower than the absolute value of the AC inputvoltage target value Vinv*, the AC/DC converter 11 u stops switching,and instead, the step-down circuit 10 d operates. The switchingmentioned here means high-frequency switching at about 20 kHz, forexample, and does not mean switching at such a low frequency (twice ashigh as the commercial frequency) that is used for performingsynchronous rectification. Even if the switching elements Q1 to Q4 areall OFF due to stop of switching in the AC/DC converter 11 u, voltagerectified through the diodes included in the switching elements Q1 to Q4is inputted to the step-down circuit 10 d. Here, in order to reduceconduction loss, it is preferable to perform synchronous rectification.

In the AC/DC converter 11 u in a case of performing synchronousrectification, through control by the control unit 12, during a periodin which the sign of current in the AC/DC converter 11 u is positive,the switching elements Q1 and Q4 are ON and the switching elements Q2and Q3 are OFF, and during a period in which the sign of current in theAC/DC converter 11 u is negative, ON and OFF of these switching elementsare inverted. The frequency of the inversion is twice as high as thecommercial frequency, and thus is very low as compared to thehigh-frequency switching frequency. Therefore, loss due to the ON/OFFinversion is extremely small.

Meanwhile, during the periods (from t1 to t2, from t3 to t4), thestep-down circuit 10 d performs step-down operation. Thin stripes shownin (d) of FIG. 21 are actually a PWM pulse train, and the duty thereofvaries in accordance with the absolute value of the AC input voltagetarget value Vinv*. As a result of the step-down operation, desired DCvoltage Vg shown in (e) of FIG. 21 is obtained.

As described above, only during a period in which the absolute value ofthe AC input voltage target value Vinv* based on AC voltage is lowerthan the DC voltage Vg, the AC/DC converter 11 u operates, and duringthe other period, switching in the AC/DC converter 11 u is stopped,whereby switching loss in the AC/DC converter 11 u can be reduced.

Similarly, only during a period in which the absolute value of the ACinput voltage target value Vinv* is higher than the DC voltage Vg, thestep-down circuit 10 d operates, and during the other period, switchingin the step-down circuit 10 d is stopped, whereby switching loss in thestep-down circuit 10 d can be reduced.

Thus, the AC/DC converter 11 u and the step-down circuit 10 dalternately perform switching operations, and when one of them operates,the other one stops switching. That is, for each of the AC/DC converter11 u and the step-down circuit 10 d, a period in which switching isstopped arises. In addition, since the AC/DC converter 11 u operates ina region other than the peak of the absolute value of the AC inputvoltage target value Vinv* and the vicinity thereof, voltage at whichthe AC/DC converter 11 u performs switching is relatively low. This alsocontributes to reduction in switching loss. Thus, switching loss in theconversion device 1R as a whole can be greatly reduced.

[Specifications of Control]

Control of the conversion device 1R can be considered to be similarcontrol obtained by reversing the direction of the control in systeminterconnection by the inverter device 1 in FIG. 2, etc. This control issuitable for, with use of the conversion device 1R which can perform thesame system interconnection as in the inverter device 1, enhancing theefficiency of the conversion device 1R also in the reversed operation.

Various values in the inverter device 1, and various values in theconversion device 1R corresponding thereto are as follows.

Ia*: a target value for input current from the commercial power system 3

Iin: a step-down circuit current detection value

Iin*: a step-down circuit current target value

Iinv*: a target value for AC input current to the AC/DC converter 11 u

Ig*: a target value for DC input current to the storage battery 2B

Ic: current flowing through the capacitor 19

Ica: current flowing through the capacitor 23

Va: a system voltage detection value

Vg: a storage battery voltage value

Vinv*: a target value for AC input voltage to the AC/DC converter 11 u

Vo*: a target value for input voltage to the step-down circuit 10 d

Pin: input power to the storage battery 2B

P_(Loss): power loss in the conversion device 1R

η: a power conversion efficiency in the conversion device 1R

Therefore, it is possible to apply the following relationshipscorresponding to the aforementioned expressions (1) to (8) for theinverter device 1 in FIG. 2, etc.

An average value <Pin> of input power Pin to the storage battery 2B,corresponding to expression (1), is represented as follows.<Pin>=<Iin×Vg>  (R1)

An average value <Ia*> of the target value for input current from thecommercial power system 3, corresponding to expression (2), isrepresented as follows.<Ia*>=<Ig*×Vg>/(η×<Va>)  (R2)

The input current target value Ia* corresponding to expression (3) isrepresented as follows.Ia*=(√2)×<Ia*>×sin ωt  (R3)

The AC input current target value Iinv* corresponding to expression (4)is represented as follows.Iinv*=Ia*−sCaVa  (R4)

The above expression (R4) is represented as follows, using a derivativewith respect to time t.Iinv*=Ia*−Ca×(dVa/dt)  (R4a)

If current flowing through the capacitor 23 is detected and the detectedcurrent is denoted by Ica, the following expression is obtained.Iinv*=Ia*−Ica  (R4b)

The AC input voltage target value Vinv* corresponding to expression (5)is represented as follows.Vinv*=Va−Za Iinv*  (R5)

The above expression (R5) is represented as follows, using a derivativewith respect to time t.Vinv*=Va−{RaIinv*+La×(dIinv*/dt)  (R5a)

As described above, the input target values (Iinv*, Vinv*) for the AC/DCconverter 11 u which are AC-side target values are set at a circuitconnection point P between the AC/DC converter 11 u and the filtercircuit 21. Therefore, as in a case of performing systeminterconnection, a point where the target values are set is moved to astage (AC/DC converter 11 u side) preceding to a circuit connectionpoint between the commercial power system 3 and the conversion device1R. By such, as it were, “reverse” system interconnection, appropriateinterconnection between AC and DC is performed.

As for the input voltage target value Vo* for the step-down circuit 10 dcorresponding to expression (6), Vgf, i.e., (Vg−Z Iin) in expression (6)is replaced with Vgr, i.e., (Vg+Z Iin), to obtain the followingexpression.Vo*=Max(Vg+ZIin,absolute value of Vinv*)  (R6)

The above expression (R6) is represented as follows, using a derivativewith respect to time t.Vo*=Max(Vg+RIin+L(dIin/dt),absolute value of Vinv*)  (R6a)

The step-down circuit current target value Iin* is represented asfollows.Iin*={(Iinv*×Vinv*)−(sCVo*)×Vo*}/(Vg+ZIin)  (R7)

The above expression (R7) is represented as follows, using a derivativewith respect to time t.Iin*={(Iinv*×Vinv*)−C×(dVo*/dt)×Vo*}/{Vg+RIin+L(dIin/dt))  (R7a)

If current flowing through the capacitor 19 is detected and the detectedcurrent is denoted by Ic, the following expression is obtained.Iin*={(Iinv*×Vinv*)−Ic×Vo*}/(Vg+ZIin)  (R7b)

In expressions (R7), (R7a), and (R7b), a term added to a product of theAC input current target value Iinv* and the AC input voltage targetvalue Vinv* is a value added in consideration of reactive power passingthrough the capacitor 19. That is, consideration of the reactive powerin addition to the power target value for the AC/DC converter 11 uallows for more accurate calculation of the value of Iin*.

Further, if power loss P_(LOSS) of the conversion device 1R is measuredin advance, the above expression (R7a) can be represented as follows.Iin*={(Iinv*×Vinv*)−C×(dVo*/dt)×Vo*−P _(LOSS)}/(Vg+ZIin)  (R7c)

Similarly, the above expression (R7b) can be represented as follows.Iin*={(Iinv*×Vinv*)−Ic×Vo*−P _(LOSS)}/(Vg+ZIin)  (R7d)

In this case, consideration of the reactive power and the power lossP_(LOSS) in addition to the power target value of the AC/DC converter 11u allows for more strict calculation of the value of Iin*.

If the electrostatic capacitance C and the power loss P_(LOSS) of thecapacitor 19 are sufficiently smaller than (Iinv*×Vinv*), the followingexpression (R8) is obtained. Iin* calculated by this expression (R8) canbe used as Iin contained in the right-hand sides of expressions (R6),(R6a), (R7), (R7a), (R7b), (R7c), and (R7d).Iin*=(Iinv*×Vinv*)/Vg  (R8)

As described above, the control unit 12 performs control so that thestep-down circuit 10 d is operated in a case of outputting voltagecorresponding to the part where the absolute value of the AC inputvoltage target value Vinv* for the AC/DC converter 11 u is higher thanthe DC voltage (Vg+Z Iin), and the AC/DC converter 11 u is operated in acase of outputting voltage corresponding to the part where the absolutevalue of the AC input voltage target value Vinv* for the AC/DC converter11 u is lower than the DC voltage (Vg+Z Iin). Therefore, a potentialdifference in power stepped up by the AC/DC converter flu can bereduced, and loss due to switching of the AC/DC converter 11 u and thestep-down circuit 10 d is reduced, whereby DC power can be outputtedwith increased efficiency.

Further, since both the step-down circuit 10 d and the AC/DC converter11 u operate based on the target values set by the control unit 12,occurrence of phase-deviation or distortion in AC current inputted tothe AC/DC converter 11 u can be suppressed even if operation isperformed so as to alternately switch the high-frequency switchingperiod between the two circuits.

In addition, as described above, the conversion device 1R can performthe same system interconnection operation as in the inverter device 1 inFIG. 2, etc. Therefore, it is possible to realize an efficientconversion device that can be used in both directions of DC/ACconversion to perform system interconnection, and AC/DC conversion.

[Others]

In FIG. 20, an example in which FETs are used as the switching elementscomposing the AC/DC converter 11 u has been shown. However, instead ofFETs, IGBTs may be used as shown in FIG. 14. In the case of IGBTs,synchronous rectification cannot be performed. Therefore, whenhigh-frequency switching of the AC/DC converter 11 u is stopped, theAC/DC converter 11 u operates as a full-bridge rectification circuit bymeans of the diodes included in the elements.

<<Supplement>>

It is desirable that, in the circuit configurations in FIG. 2, FIG. 14,and FIG. 20, SiC elements are used for at least one of the semiconductorswitching elements included in the DC/DC converter 10, and thesemiconductor switching elements included in the DC/AC inverter 11 (orAC/DC converter 11 u).

In the above conversion device 1, switching loss in the semiconductorelements and iron loss in the DC reactor 15 and the AC reactor 22 can bereduced by decrease in the number of times of high-frequency switching,but conduction loss in the semiconductor elements cannot be reduced. Inthis regard, using SiC elements as the semiconductor elements enablesreduction in the conduction loss. Therefore, if SiC elements are usedfor the conversion device 1 controlled as described above, a highconversion efficiency can be achieved by the synergistic effecttherebetween.

It is noted that the embodiments disclosed herein are merelyillustrative in all aspects and should not be recognized as beingrestrictive. The scope of the present invention is defined by the scopeof the claims and is intended to include meaning equivalent to the scopeof the claims and all modifications within the scope.

REFERENCE SIGNS LIST

-   -   1 inverter device    -   1R conversion device    -   2 photovoltaic panel    -   2B storage battery    -   3 commercial power system    -   10 step-up circuit (DC/DC converter)    -   10 d step-down circuit (DC/DC converter)    -   11 inverter circuit (DC/AC inverter)    -   11 u AC/DC converter    -   12 control unit    -   15 DC reactor    -   16 diode    -   17 first voltage sensor    -   18 first current sensor    -   19 capacitor (smoothing capacitor (second capacitor))    -   21 filter circuit    -   22 AC reactor    -   23 capacitor (output smoothing capacitor (first capacitor))    -   24 second current sensor    -   25 second voltage sensor    -   26 capacitor    -   27 third voltage sensor    -   30 control processing unit    -   32 step-up circuit control unit    -   33 inverter circuit control unit    -   34 averaging processing unit    -   41 first calculation section    -   42 first adder    -   43 compensator    -   44 second adder    -   51 second calculation section    -   52 third adder    -   53 compensator    -   54 fourth adder    -   P circuit connection point    -   Q1 to Q4, Qb switching element

The invention claimed is:
 1. A high-frequency switching type conversiondevice that converts DC power provided from a DC power supply, to ACpower and supplies the AC power to a load, the conversion devicecomprising: a filter circuit connected to the load and including an ACreactor and a first capacitor; a DC/AC inverter connected to the loadvia the filter circuit; a DC/DC converter provided between the DC powersupply and the DC/AC inverter; a second capacitor provided between theDC/AC inverter and the DC/DC converter; and a control unit configured tocontrol the DC/DC converter and the DC/AC inverter to alternately have astop period of high-frequency switching, the control unit beingconfigured to set a current target value for the DC/DC converter tothereby be synchronized with current of the AC power, based on voltageof the AC power, voltage variation due to current flowing through the ACreactor and an impedance of the AC reactor, reactive currentsrespectively flowing through the first capacitor and the secondcapacitor, and voltage of the DC power, wherein, in a case where anoutput current target value for the load is Ia*, an electrostaticcapacitance of the first capacitor is Ca, a voltage value of the ACpower is Va, voltage on the DC power supply side is V_(DC), and aLaplace operator is s, the control unit sets an AC output current targetvalue Iinv* for the DC/AC inverter at a circuit connection point betweenthe filter circuit and the DC/AC inverter.
 2. The conversion deviceaccording to claim 1, wherein in a case where an output current targetvalue for the load is Ia*, an electrostatic capacitance of the firstcapacitor is Ca, a voltage value of the AC power is Va, voltage on theDC power supply side is VDC, and an Laplace operator is s, the controlunit sets an AC output current target value Iinv* for the DC/AC inverterat a circuit connection point between the filter circuit and the DC/ACinverter, as follows “this should be” in the case where the outputcurrent target value for the load is Ia*, the electrostatic capacitanceof the first capacitor is Ca, the voltage value of the AC power is Va,voltage on the DC power supply side is V_(DC), and the Laplace operatoris s, the control unit sets the AC output current target value Iinv* forthe DC/AC inverter at the circuit connection point between the filtercircuit and the DC/AC inverter, as followsIinv*=Ia*+sCaVa, in a case where an impedance of the AC reactor is Za,the control unit sets an AC output voltage target value Vinv* for theDC/AC inverter at the circuit connection point, as follows:Vinv*=Va+ZaIinv*, the control unit sets the greater one of the voltageV_(DC) and an absolute value of the AC output voltage target value Vinv*for the DC/AC inverter, as an output voltage target value Vo* for theDC/DC converter, and in a case where an electrostatic capacitance of thesecond capacitor is C, the control unit sets a current target value Iin*for the DC/DC converter, as follows:Iin*={(Iinv*×Vinv*)+(sCVo*)×Vo*}/V _(DC).
 3. The conversion deviceaccording to claim 2, wherein the DC/DC converter includes a DC reactor,and in a case where voltage of the DC power supply is Vg, an impedanceof the DC reactor is Z, and a current value of the DC/DC converter isIin, (Vg−ZIin) is used as the voltage V_(DC).
 4. The conversion deviceaccording to claim 3, wherein the current value Iin of the DC/DCconverter is set to a detection value of a current sensor or acalculation value obtained by Iinv*×Vinv*/Vg.
 5. The conversion deviceaccording to claim 1, wherein the DC/AC inverter is controlled based oncomparison between a reference value based on a target value and adetection value of AC output current of the DC/AC inverter, and anoutput voltage target value for the DC/DC converter, and the DC/DCconverter is controlled based on comparison between a reference valuebased on a current target value and a detection value of the DC/DCconverter, and the output voltage target value for the DC/DC converter.6. The conversion device according to claim 1, wherein the load is acommercial power system.
 7. The conversion device according to claim 1,wherein the DC power supply is a DC load, and the load is an AC powersupply, and power is supplied from the AC power supply to the DC load.8. The conversion device according to claim 1, wherein a SiC element isused for at least one of semiconductor switching elements included inthe DC/DC converter and the DC/AC inverter.
 9. The conversion deviceaccording to claim 2, wherein the DC/AC inverter is controlled based oncomparison between a reference value based on a target value and adetection value of AC output current of the DC/AC inverter, and anoutput voltage target value for the DC/DC converter, and the DC/DCconverter is controlled based on comparison between a reference valuebased on a current target value and a detection value of the DC/DCconverter, and the output voltage target value for the DC/DC converter.10. The conversion device according to claim 3, wherein the DC/ACinverter is controlled based on comparison between a reference valuebased on a target value and a detection value of AC output current ofthe DC/AC inverter, and an output voltage target value for the DC/DCconverter, and the DC/DC converter is controlled based on comparisonbetween a reference value based on a current target value and adetection value of the DC/DC converter, and the output voltage targetvalue for the DC/DC converter.
 11. The conversion device according toclaim 4, wherein the DC/AC inverter is controlled based on comparisonbetween a reference value based on a target value and a detection valueof AC output current of the DC/AC inverter, and an output voltage targetvalue for the DC/DC converter, and the DC/DC converter is controlledbased on comparison between a reference value based on a current targetvalue and a detection value of the DC/DC converter, and the outputvoltage target value for the DC/DC converter.